docs: def _start_stop(item):
      """Find the start and stop indices of a __getitem__ item.
  
      This is used only by ConcatenatedArrays.
  
      Only two cases are supported currently:
  
      * Single integer.
      * Contiguous slice in the first dimension only.
  
      """
      if isinstance(item, tuple):
          item = item[0]
      if isinstance(item, slice):
          # Slice.
          if item.step not in (None, 1):
              raise NotImplementedError()
          return item.start, item.stop
      elif isinstance(item, (list, np.ndarray)):
          # List or array of indices.
          return np.min(item), np.max(item)
      else:
          # Integer.
          return item, item + 1